The correct command to add a VLAN with a specific ID to a FlexConnect group in a Cisco Wireless LAN Controller (WLC) is 'config flexconnect group BranchA-FCG vlan 30 add'. This command specifies the FlexConnect group name 'BranchA-FCG' and the VLAN ID '30', followed by the action 'add', which is consistent with Cisco's CLI syntax for WLC configuration. References: CCNP Enterprise Wireless Design ENWLSD 300-425 and Implementation ENWLSI 300-430 Official Cert Guide